Still more increasing tendency of duplication speed has recently been found in the electrophotography due to the increase of information to be treated.
Consequently, the heat quantity transferred from hot fixing rolls to toner is less at high duplication speed than at low duplication speed. A remarkable decrease in the surface temperature of fixing rolls is also caused by the heat removal to copying papers. Therefore the toner is required to be fixed at lower temperatures and also to be free from offset phenomenon at these fixing temperatures. In order to obtain a sharp image, improvement of resin has been conducted with respect to hot melt properties such as fixing ability at lower temperatures and offset resistance, as well as electrostatic characteristics of the toner.
For example, several patents have been known. Japanese Patent Publication No. 6895/1980 discloses a method for providing a toner having a good offset resistance by using a resin having a weightaverage molecular weight/number average molecular weight ratio of 3.5-40 and a number average molecular weight of 2,000-30,000. Japanese Patent Laid-open No. 144,446/1975 describes a method for improving the fixing ability by adding a small amount of plasticizers such as phthalic acid diester into a toner having a good blocking and offset resistance. Japanese Patent Laid-open No. 101,031/1974 discloses a method for extending the range of fixing temperatures by using a crosslinked resin and for employing a toner which is offset resistant even at relatively high fixing temperatures. Besides patents are known as a countermeasure for providing the high electrostatic charge characteristics for the toner. For example, Japanese Patent Publication 40,183/1983 discloses a method for using aliphatic unsaturated carboxylic acids such as methacrylic acid as a component of the resin. Japanese Patent Laid-open No. 93,457/1984 discloses a method for providing charge stability together with the high electrostatic charge characteristics by adding a charge control agent composed of metal containing dyestuff as a toner ingredient.
Furthermore, Japanese Patent Laid-open No. 16,144/1981 relevant to U.S. Pat. No. 4,499,168 describes a method for providing a magnetic toner which is excellent in the fixing ability and impact resistance by employing the resin having the maximum value of molecular weight in a specific molecular weight region.
As above mentioned, the heat quantity provided from the hot fixing rolls is less at the high duplication speed than at the low-duplication speed. A marked decrease in the surface temperature of fixing rolls is also caused by the heat removal to the copying papers. Therefore it is necessary fixing with a smaller quantity of heat. Smaller molecules having lower glass transition temperature (hereinafter abbreviated as Tg) are required for melting with low calory. Excess lowering of Tg, however, causes blocking and there is naturally a lower limit for the Tg. The smaller molecules are assumed to reduce their melt viscosity more rapidly, enhance flowability of the resin at lower temperatures, and improve the fixing ability. Too small molecules, however, lead to lowering of Tg and occurrence of blocking problems.
On the other hand, as a result of increase in the duplication speed and numbers of copying papers, the duplicated images are expected to have the same quality from the 1 st to the dozens of thousandth sheet in addition to have a sharp image and perfect fixation of the toner to the paper.
Conventional methods for the improvement of offset resistance and low temperature fixation are related to the problems occurring after adhesion of the toner to the paper. These methods are important and yet not considered upon the requirement for adhering the toner in advance on each copying paper uniformly and at a constant concentration. The electrostatic charge characteristics of the toner is an important factor for the determination of toner quantity adhering on the paper and controls the image concentration. On the other hand, in the two component type developers for example, triboelectrostatic charge generates by the friction of the toner with carrier. Consequently partial destruction of the toner causes separation of resin particulates, particulate powder of coloring agents such as carbon black, or powder of its aggregates. These particulates are different from the employed toner particles in diameter and shape, ratio of the resin to coloring agents, molecular weight caused by destruction of the binder resin molecules etc. Thus these particulates exhibit different behavior on the electrostatic charge characteristics. Consequently scattering of the particulates, make a dirty mark in the copy machine and increase in the background concentration of image are generated as the increase in numbers of copying papers. As a result, the duplicated image cannot be maintained in the same quality.
In addition, the particulates are absorbed on the carrier and result in the variation of triboelectrostatic charge which leads to alter the image concentration. Accordingly the consistent maintenance of a constant image concentration cannot be achieved. Aforesaid Japanese Patent Laid-open No. 16,144/1981 describes that above mentioned destruction of the toner results from the lack of hardness in the binder resin and defines to have the maximum value in a molecular weight region of 10.sup.5 -2.times.10.sup.6. The correlation between presence of the maximum value and hardness is not clear. Furthermore the maximum value is not essential for preventing the destruction of toner even though the maximum value exists in this molecular weight region.
On the other hand, the method of Japanese Patent Laid-open No. 101,031/1974 is an effective technique for improving resin strength and yet may cause poor flowability in the melted stage by the hot rollers because crosslinked binder resin, that is, gel is contained in the toner. Consequently, irregular gloss emerges on the duplicated image, particularly in the solid block parts of the duplicate, and remarkably damages the quality of image.
The methods of Japanese Patent Publication No. 40,183/1983 and Japanese Patent Laid-open No. 93,457/1984 are considered excellent for controlling the quantity of electrostatic charge in the initial stage of duplication. The toner, however, is not guaranteed for its strength at all and has not yet been solved the problem of its destruction caused by increase in the numbers of copying papers.